1. Field of the Invention
The present invention relates to a novel organic compound and an electroluminescent device using the same.
2. Description of the Related Art
Pope et al., first discovered an electroluminescence (EL) of an organic material, that is, single-crystal anthracene in 1963 (J. Chem. Phys., 38, 2042 (1963)). Helfinch and Schneider succeeded in observing relatively strong EL in an injection EL material containing a solution system having a high injection efficiency in 1965 (Phys. Rev. Lett., 14, 229 (1965)). Many studies of organic luminescent materials containing conjugated organic hosts and conjugated organic activators having fused benzene rings have been disclosed in U.S. Pat. Nos. 3,172,862, 3,173,050, and 3,710,167; J. Chem. Phys., 44, 2902 (1966); J. Chem. Phys., 58, 1542 (1973); and Chem. Phys. Lett., 36, 345 (1975). Examples of disclosed organic hosts include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzpyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyls, trans-stilbene, and 1,4-diphenylbutadiene. Examples of disclosed activators include anthracene, tetracene and pentacene. Since these organic luminescent materials are provided as single layers having a thickness of more than 1 μm, a high electric field is required for luminescence. Under these circumferences, thin film devices formed by a vacuum evaporation process have been proposed (for example, Thin Solid Films, 94, 171 (1982); Polymer, 24, 748 (1983); and J. Appl. Phys., 25, L773 (1986)). Although the thin film devices are effective for reducing the driving voltage, their luminance is far from a level for practical use.
Tang et al. developed an EL device having a high luminance for a low driving voltage (Appl. Phys. Lett., 51, 913 (1987) and U.S. Pat. No. 4,356,429). The EL device is fabricated by depositing two significantly thin layers, that is, a charge transport layer and a luminescent layer, between a positive electrode a negative electrode by a vacuum evaporation process. Such layered organic EL devices are disclosed in, for example, Japanese Patent Laid-Open Nos. 59-194393, 63-264692, and 3-163188, U.S. Pat. Nos. 4,539,507 and 4,720,432, and Appl. Phys. Lett., 55, 1467 (1989).
Also, an EL device of a triple-layered structure having independently a carrier transport function and a luminescent ability was disclosed in Jpn. J. Appl. Phys., 27, L269 and L713 (1988). Since the carrier transportability is improved in such an EL device, the versatility of possible dyes in the luminescent layer is considerably increased. Further, the device configuration suggests the feasibility of improved luminescence by effectively trapping holes and electrons (or excimers) in the central luminescent layer.
Layered organic EL devices are generally formed by vacuum evaporation processes. EL devices having considerable luminance are also formed by casting processes (as described in, for example, Extended Abstracts (The 50th Autumn Meeting (1989), p. 1006 and The 51st Autumn Meeting (1990), p. 1041; The Japan Society of Applied Physics). Considerably high luminance is also achieved by a single-layered mixture-type EL device, in which the layer is formed by dip-coating a solution containing polyvinyl carbazole as a hole transport compound, an oxadiazole derivative as an electron transport compound and coumarin-6 as a luminescent material (as described in Extended Abstracts (The 38th Spring Meeting (1991), p. 1086; The Japan Society of Applied Physics and Related Societies).
As described above, the organic EL devices have been significantly improved and have suggested feasibility of a wide variety of applications; however, these EL devices have some problems for practical use, for example, insufficient luminance, a change in luminance during use for a long period, and deterioration by atmospheric gas containing oxygen and humidity. Further, wavelengths of electroluminescent light are limited, hence it is difficult to precisely determine hues of blue, green and red colors for use in full-color displays.
Japanese Patent Laid-Open No. 5-25473 discloses an electroluminescent device using an organic compound represented by the following general formula (A) as a hole transfer material: wherein R1 is an alkyl or aralkyl group, and R2, R3, R4 and R5 are each a hydrogen or halogen atom or an alkyl or alkoxyl group. The organic compound is capable of forming an EL device which can drive at a low voltage and which has high luminance and high durability.
According to the results by the present inventors, however, a light emitting diode (LED) and a display using such an EL device are inevitably heated during operation. Since organic compounds represented by the formula (A) have a relatively low melting temperature and a glass transition temperature, their durability is insufficient due to deterioration by heat.